1. Field of the Invention
The present invention relates to a machine tool numerical controller which is capable of checking as to the interference of a cutting tool with a workpiece the relative movement between which is controlled in accordance with a numerical control program.
2. Description of the Prior Art
Generally, when a workpiece is machined in a numerically controlled machine tool called a "machining center", programming errors involved in the numerical control program (hereafter referred to as an "NC program") may cause a cutting tool to be brought into engagement with the workpiece at a rapid feed rate, thus causing damage to the workpiece and the cutting tool. To avoid this, an initial program debugging must be performed upon completion of preparation of the NC program and in advance of the initial operation of the machine tool in a continuous run mode according to the prepared NC program. The program debugging is done by actually operating the machine tool in a single block mode in accordance with the NC program so as to ascertain whether or not the cutting tool is brought into engagement with the workpiece at a rapid feed rate.
Such program debugging is however impossible to perform while the machine tool is in operation for machining another workpiece in accordance with a different NC program, and therefore, disadvantageously leads to inefficient use of the machine tool. Further, during the program debugging, a programmer or operator has to decide whether or not the cutting tool will come into contact with the workpiece, before the contact actually occurs. During the program debugging, the machine tool must therefore be moved at a considerably slower feed rate than that given in the NC program, and a long period of time is spent for such program debugging. More particularly, workpieces machined by machining centers have protrusions and cavities of various shapes and are more complicated in shape than those machined by lathes or turning machines Thus, the programming of NC programs for the workpieces machined by machining centers is more likely to involve errors, and a method for checking the NC programs for such errors is presently unavailable except for actually operating the machining centers in accordancwe with the NC programs.
There has also been technology developed for checking whether or not a workpiece interferes with other machine tool components such as, for example, a spindle head, a tool support and the like when the tool is directed against the workpiece to effect machinings thereon. However, the conventional interference checking technology is designed to check the interference between two relatively movable objects which should not normally occur, and an interference space is set to define an area in which the movable objects are absolutely prevented from entering. Accordingly, it is impossible to use the conventional interference checking technology to check for interference or rapid feed engagement between a tool and a workpiece, because it is impossible to direct the tool against the workpiece.